In dentistry, missing teeth are restored by bridges or dentures. In this case, when there is a remaining tooth, a clasp (spring) is applied to that tooth.
Such a clasp is usually formed of an about 1 mm-diameter dental wrought alloy wire made of gold alloy, gold-platinum alloy, gold-silver-palladium alloy, nickel-chromium alloy and cobalt-chromium alloy, etc. That wire is bent along the shape of the remaining tooth in the substantially round form, one end is located in the under-cut of that tooth, and the remainder end is inserted into the denture base for stabilization. Therefore, it is required to bend the dental wrought alloy wire in association with the shape of a desired clasp. However, to form the alloy wire into a compatible clasp with good dimensional accuracy is not easy, and requires rather skill. More recently, such a compatible clasp has been prepared by dental precision casting from dental casting alloys such as gold alloy, gold-platinum alloy, gold-silver-palladium alloy, nickel-chromium alloy, cobalt-chromium alloy, etc.
However, the clasps generally obtained by casting presented a problem that they suffered easier casting defects and were poorer in durability as compared with those obtained by bending of dental wrought alloy wires. Especially with nickel-chromium alloy or cobalt-chromium alloy having so high melting points that they suffer easier casting defects, breakage accidents of clasps occur frequently.
Since clasps undergo repeated elastic deformation during attachment or detachment of a denture or due to a large occlusal force exerted during occlusion. Thus, when they receive an occlusal force that is too large to allow the amount of deformation to exceed the elastic limit, they deform permanently. A lowering of the maintaining force between the dentures and clasps then takes place, so that they lose their own function. Even when such deformation is within the elastic limit, they may break due to fatigue while being exposed to repeated deformation.